PROJECT SUMMARY Osteoarthritis (OA) is the most common form of arthritis in the United States and a leading cause of physical disability in older individuals. There are no approved pharmacologic treatments available beyond those for symptomatic relief. Thus, identifying new treatment targets is critical to reducing the burden of OA. While age is one of the strongest predictors of OA, the disease mechanisms that underlie the relationship between cellular and molecular hallmarks of aging and OA are unclear. The long-term goal of this ancillary study is to identify potential new targets for OA therapy by identifying key age-associated biological mechanisms underlying OA pathogenesis. The objective of this project is to determine the role of epigenetic changes to genes in key aging pathways on OA. The central hypothesis is that aging is associated with epigenetic modifications affecting DNA methylation levels in immune blood cells (i.e., `inflammaging') and will be associated with greater burden of disease across commonly affected synovial joints, including the hands, knees and hips. Epigenetic modifications that affect downstream changes in gene and protein expression without altering the DNA sequence may provide important insights into OA pathogenesis that are not captured by information on genetic or environmental risk factors. The contribution of age-related epigenetic mechanisms to OA burden will be assessed in the MOST study, an ongoing well-characterized cohort of individuals with and without early stage knee OA. The pending reexamination of MOST subjects presents a unique, time-sensitive opportunity to obtain contemporaneous measures of epigenetic markers and OA burden that includes assessments of hand OA from radiographs. New acquisition of hand radiographs will be combined with the ongoing radiographic imaging of knees and hips to constitute a multiple joint OA phenotype. The acquisition of hand radiographs and genome-wide DNA methylation will make it possible for the first time in MOST to study the relation of epigenetic factors with multiple joint OA. Aims of the study include 1) determining whether epigenetic age acceleration is associated with multiple joint OA by assessing the difference between chronological age and epigenetic molecular age, then relating this to multiple joint OA and 2) identifying novel epigenetic markers associated with multiple joint OA by performing an epigenome-wide association study, then pathway analyses and hierarchical clustering to identify whether differentially methylated loci and OA subtypes are enriched for biological pathways related to aging. The expected outcome is identification of age-associated epigenetic mechanisms that underlie OA pathogenesis, which is needed to identify relevant OA treatment targets and biomarkers that could be used to identify at-risk individuals earlier in the disease process before the onset of structural changes in the joint. The proposed ancillary study will also provide valuable epigenetics data that will lay the groundwork for future integrative studies of genetics/genomics in the MOST parent study, greatly enhancing its scientific value to the medical community.